Fail-safe apparatus and technique



Sept. 17, 1957 w. G. ROWELL 2,307,010

FAIL-SAFE APPARATUS AND TECHNIQUE Filed May a, 1956 2 4 LOAD SUPPRESSION DEVICE CONDITION SYSTEMTlgESPONSlVE J f 6 SENSOR SENSOR SIGNAL 14 |5-- ]\2'L R I LOAD l FlG.l

s 9 a 2L 5 D1 SYSTEM RESPON' LOA INDE ENOEN COND'TION P T SIVE TO SENSOR SUPPRESSION LOAD SENSOR SIGNALMODIFIEB SIGNAL DEVICE FIG. 2

7 LOAD 6 QRYI 9 c J. "1 RY2 c2 8 11 I50. A T

RYZ.

FIG.4

IN VEN TOR. WILLIAM G. ROWELL BYE AT TORNEYS United States Patent FAIL-SAFE APPARATUS AND TECHNIQUE William G. Rowell, Quincy, Mass., assignor toScnliy Signal Company, Melrose, Mass., a corporation of Massachusetts Application May 8, 1956, Serial No. 583,569

16 Claims. (Cl. 340-213) The present invention relates to electrical systems, and, more particularly, to fail-safe electrical circuits in which a load can not be falsely operated through failure of any of the components of the circuits.

In copending application Serial No. 375,224, entitled Checking Method and System and filed August 19, 1953, now Patent No. 2,798,213, systems and techniques for producing the fail-safe operation of electrical, electronic, mechanical and electro-mechanical systems are set forth. Some of these systems and techniques are described, also, in an article entitled Fail-safe gets new meaning, by William G. Rowell and A. B. Van Rennes, appearing on pages 79 through 81 of Control Engineering, March 1956, and in an article entitled Fail-safe monitoring, by William G. Rowell, appearing on pages 28 through 31 of Electronic Design, March 1, 1956. In the output of such systems, there is usually provided a switching apparatus that recurrently operates to maintain a slowly responding load, which may assume the form of II. slowly acting output or load relay, in either an energized or de-energized state, so long as the switching operation in the output continues. Failure of any of the electrical or other components in the system would produce improper operation of the output switching apparatus and, in turn, would result in de-energizing or energizing the output or load relay, indicating a failure in the system.

An object of the present invention is to provide a new and improved switching apparatus of the above-described character that is particularly adapted for operation in the fail-safe systems above-described.

A further object is to provide a more generally usable electrical switching apparatus that is adapted for recurrent operation and that, through such operation, controls the ultimate response of an electrical load and insures that any failure in the switching apparatus or the associated circuitry will not result in falsely holding the load energized or de-energized, whichever condition it is normally desired to maintain. The term fail-safe as used in the specification and claims is intended to connote that integrity failure of any of the components involved in the circuits will not result in false operation of the ultimate output load.

An additional object of the present invention is to provide a new and improved technique for fail-safe operation that is adapted for practice with a wide variety of different types of apparatus including electrical, electronic, mechanical and electro-mechanical devices.

Still an additional object is to provide a new and improved system of the character described which embodies presently available conventional inexpensive components, and does not necessitate the utilization of specialized electrical components.

A further object is to provide such a switching apparatus that is particularly adapted for use in installations where the terminal power supply is either direct or alternating current.

While various proposals have heretofore been made for providing switching systems that recurrently operate to store energy and then to deliver energy to an output load, many such systems, with the exception of some of those described in the said copending application and the above-mentioned articles, are subject to the diificulty that the associated electrical switching contacts may become shorted, fused together or otherwise damaged, and other similar circuit failures may take place such that the ultimate load may become falsely operated. Some prior-art devices of this character embody load relays that must be of special construction, such as those of the polar type having special means whereby the relay is slow-acting in the release direction only. Such devices require mechanical dash-pot arrangements and the like in order to obtain the essential slow action in one direction only, which add to the complexity and cost of the device.

In accordance with the present invention, on the other hand, simple conventional electrical components may be utilized to obtain complete fail-safe operation irrespective of short-circuited or fused-together parts, or other damage or failure in the system. In summary, the technique or method and system underlying the present invention provides for switching means that is adapted to occupy alternate positions and that is repetitively operated at a predetermined repetition rate or rates repetitively to alternate between these positions. Means is provided for supplying direct-current potential of a predetermined magnitude. An electric circuit operates when the switching means occupies one of its positions to connect the direct-current potential supplying means to pctential-storing means. A slow-response direct-currentoperated load is provided that is adapted to respond after the elapse of a period greater than the period or periods of the said repetition rate or rates and that is operable only in response to direct-current potential of magnitude greater than the said predetermined magnitude. A further electric circuit operates when the switching means occupies the alternate position to connect the potentialstoring means and the supplying means in series and to the load. Preferred constructional details are hereinafter disclosed.

The invention will now be explained in connection with the accompanying drawings, Fig. 1 of which is a block diagram of a system embodying the present invention as a preferred output component thereof;

Fig. 2 is a block diagram of a modification; and

Fig. 3 is a circuit diagram of the invention in preferred form, and Fig. 4 is a similar diagram of a modification.

Referring, first, to Fig. l, a system is therein disclosed in which an element 2, labelled Condition Sensor, is provided for detecting any signal, event or condition that it is desired to monitor or receive and for passing an electrical signal indication thereof to a system 4, labelled System Responsive to Sensor Signal. The condition sensor 2 may comprise any kind of detecting device, such as, for example, a light-sensing element, a radiation-sensing element, a sound-sensing element, a heat-sensing element, an electro-mechanical transducer, a forceor pressure-sensing element, a currentor voltage-sensing element, or any other type of monitoring or receiving device, as explained in said application and the said articles. The system 4 may be any kind of receiving, amplifying or transmission system and the like. The connection from the condition sensor or detector 2 to the left-hand side or input of the system 4 is shown provided by means of a conductor 1, a switch 3 and a conductor 5. The signal, event or condition detected by the condition sensor 2 and thus fed to the input of the system 4 may be termed the principal signal. There will therefore appear at the right-hand or output side of the system 4, along the conductor '7, an output signal that results from the transmission of the principal signal through the system 4 between its input and output. Connected with the output conductor 7 is a load-suppression device 6 indicated schematically as comprising a relay RY1, the armature of which, indicated by the vertical dotted line 11, controls not only the before-mentioned switch 3, but, also, a further switch generally represented by the numeral 15. The switch 15 is schematically illustrated as adapted either to make or break connection with a further conductor 9 that connects to an ultimate output termination 8, labelled Load. This schematic representation is intended to indicate that when the relay RY1 is, for example, energized, the switch 15 may connect with conductor 9 and normally energize the load 8. When, however, the relay RY1 is de-energized, the switch 15 may disconnect from the conductor 9. It is to be understood, of course, that the converse condition of normally maintaining the load 8 de-energized and then energizing the same, may, if'desired, be employed. Continuing with the assumption of normal energization of the load 8 upon energization of the relay RY1, the advent of the principal signal in the output conductor 7, resulting in the energizing of the coil of the relay RY1, also causes the relay armature 11 to open the switch 3. This opens the connection between the conductors 1 and and thus the connection between the condition sensor 2 and the system 4. Such a break in the input circuit results in modifying, modulating or chopping the principal signal in the input to the system 4 so that the principal signal no longer appears in the output conductor 7. The relay RY1 accordingly becomes thereupon de-energized and its armature again closes the switch 3 thus to restore the feeding of the principal signal to the input of the system 4 from the condition sensor 2 by way of the conductor 1, the closed switch 3 and the conductor 5. This feed-back or reaction from the output to the input of the system 4 is thus caused to occur periodically at a predetermined repetition rate or rates, providing, in effect, a chopping checking signal which accompanies the principal signal flowing through the system 4 between its input and output. So long as this oscillating reactive effect between output and input takes place, the relay RY1 will continue to recover the checking signal at the said repetition rate or rates and continue to modify the principal signal at that rate or rates. The system 4 will thus be maintained in periodic checking operation. Simultaneously therewith, the switch 15 will periodically connect to the conductor 9 and disconnect therefrom, thus, as explained in the said application and articles, and as hereinafter more fully explained, periodically feeding energy to keep the ultimate output load 8 energized.

The load 8 is a slowly responsive device that is adapted to respond only in a period of time greater than the said repetition rate or rates of the before-mentioned signal modification, so that only in the event of the loss of the checking signal or chopping modification, will the load 8 respond to produce an indication of failure in the system.

It is not necessary, though it is preferred for purposes of simplicity and economy, that the feed-back control of Fig. 1 be utilized. As explained in the previously mentioned application and articles, the checking signal may be introduced by means of an independent signal modifier 3 placed, for example, between the condition sensor 2 and the input of the system 4. Again, however, the recovery of the checking signal in the output load-suppression device 6, will maintain the load 8 energized so long as the checking signal, produced by the independent signal modifier 3', Fig. 2, accompanies the prin cipal signalto the output of the system 4. If desired, moreover, the signal reaching the condition sensor 2 may already be provided with a checking-signal modification;

In Fig. 3, a preferred type of load-suppression device 6 is illustrated, comprising the relay RY1 for receiving the periodically modified principal signal by means of the conductor 7. The switch 15 of Fig. 3 is more specifically defined than in the schematic showing of Fig. 1, comprising two switch members 15a and 15b synchronously operated by the movement of the armature 11. A restoring spring 13 normally holds the respective switch members 15a and 15b in the illustrated lower position, in engagement with respective contacts A and E. When the armature 11 moves upward in response to energization of the relay RY1, it pivots the switches 15a and 15b upward about the pivot contact points B and D to an upper position of operation. The switch member 15a then makes electrical contact between the pivot point B and the switch contact C. The switch member 15b, on the other hand, then makes electrical contact between the pivot point D and the switch contact F.

When the switch members 15a and 15b are disposed in the illustrated lower position, the storing capacitor C1 charges or stores direct-current potential from directcurrent voltage-supply terminals, shown at and through the switch members 15:: and 15b. The capacitor C1 will store a direct-current potential substantially equal to the predetermined magnitude of the voltage at the terminals and say volts, more or less. Upon energization of the relay RY1 and upward movement of the armature 11, the capacitor C1 is connected in series circuit with the voltage-supply terminals and and supplies to the load 8, by way of conductor 9, the direct-current potential of predetermined magnitude, stored by the capacitor C1 supplemented by the additional direct-current potential available at the terminals and namely, a total of 220 volts, in the above illustration. In accordance with the present invention, the load 8 is operable only with direct-current voltages of magnitude in excess of the said predetermined voltage magnitude available from the terminals and alone. The load 8 is shown in the form of a conventional slowrelease direct-current-operated relay RYZ, operable at, say 200 volts, in the above example, and designed to provide a high impedance to alternating-current energy, as is customary with conventional direct-current relays. The load 8 will thus become energized by the supplemented direct-current potential in the supply circuit traceable from the terminal to pivot point D, across the switch member 151; to contact F and the top electrode of capacitor C1; then, from the bottom electrode of capacitor C1 to the lower terminal of relay RY2, and thence from the upper terminal of relay RYZ by conductor 9 to contact C and through switch member 15a to the terminal. The ultimate load relay RYZ is shunted by a conventional holding capacitor C2.

When the relay RY1 is energized to feed energizing direct-current potential to the load relay RYZ, however, the armature 11 also causes the switch 3 to break contact between conductors 1 and 5 and thus to chop or modify the input principal signal to the system 4, de-energizing the relay RY1 and returning the switch members 15a, 15b and 3 to their illustrated positions. Since, as before stated, the relay RY1 is thus periodically energized and de-energized at the repetition rate or rates of the checking signal before-referred to, the switching members 15:: and 15b are alternately operated at the said rate or rates, thus alternately to store energy in the capacitor C1 and thence to feed or deliver the stored energy supplemented by the potential at the supply terminals and to the slow-response relay RY2. The response of the directcurrent relay RY2 is adjusted to permit, for example, a release response after the lapse of a period of time greater than the period or periods of the before-mentioned repetition rate or rates. So long as the switch members 15a and 1512 are recurrently operated at the stated rate or rates, the load relay RYZ will remain effectively energized. The frequency, of course, may be determined by the values of C1, C2, and the resistance of the load-relay coil RYZ with the applied voltage.

It will be evident from inspection that the system of Fig. 3 is completely fail-safe, in that the load 8, no matter what form it may assume, provided it has the beforedescribed characteristics, will not be falsely energized if the integrity of any of the components of the circuit is lost. It is to be understood that the ultimate load 8 may assume other forms than the relay RY2, as explained in the said application and the said articles. It may, for example, comprise an indicating or a controlling apparatus, or it may comprise a meter or any other suitable type of direct-current load operable at a potential in excess of that of the voltage supply source The load relay RY2, moreover, may itself have contacts, not shown, as discussed in the said'articles and in the said application, which, in turn, can operate to control further apparatus. It is also to be understood that the voltage supplied to the load 8 may not only be substantially doubled by the use of the capacitor C1 and the seriescircuit feed effected by the switching members 150 and 15b, but it may also be tripled, quadrupled or otherwise increased as with the aid of further capacitors and switching means. The supply voltage from the terminals furthermore, need not itself be used to supplement the volatge fed to the relay RY2, as later discussed in connection with the embodiment of Fig. 4 in which a further storage capacitor C3 in effect serves to supply voltage to charge the capacitor C1 and to supplement the voltage fed by the same to the load 8. When the source is used in series with the condenser C1 to energize the load 8, however, as in Fig. 2, the advantage is obtained of permitting a more rapid release-response of the relay RY2 in the event of power failure than could be obtained when the capacitor C3 of Fig. 4 is employed.

Where only alternating-current potential is available, the system of Fig. 4 may be employed. The system of Fig. 4 is similar to that of Fig. 3, but it additionally employs the before-mentioned further capacitor C3 that, when the switch members 15a and 15b are in the illustrated down position, is charged through a further switch member 150, also operated by the armature 11 of the relay RY1, from alternating-current supply terminals 17 and 19 through a limiting impedance, shown as a resistor R, and a rectifier or converter S. The storage capacitor C3, in turn, through its parallel connection with the capacitor C1 when the switch members 15a and 15b engage the respective contacts A and E, as shown, serves the same function as the direct-current source of Fig. 3 when the switch members 15a and 1512 are moved to their upper position in engagement with the respective contacts C and F. In such upper position, as explained in connection with the system of Fig. 2, the direct-current voltage at the pivot points B and D, produced by the charged capacitor C3 in Fig. 4, adds in series with the direct-current voltage stored in the capacitor C1 to energize the load 8. The capacitor C3 may, in fact, be the conventional smoothing capacitor of a direct-current power supply system R, S, C3, and serves the additional function of supplementing the voltage fed to the load 8 by the capacitor C1 when the relay RY1 is energized and moves the switch members 15a and 15b to the upper position. The system of Fig. 4, like that of Fig. 3, is completely fail-safe.

In the case where the circuits of Figs. 3 and 4 are utilized in the system of Fig. 2, moreover, the switches 3, shown dotted in Figs. 3 and 4 to illustrate operation in the system of Fig. 1, need not be employed. While the invention has been described in connection with its important application to the fail-safe systems of Figs. 1 and 2, the invention is of broader utility in any applications 'where periodic or switched feeding of a load is desired. The conductor 7 feeding the relay RY1 may therefore comprise any kind of recurrent signal source for periodically energizing the relay RY1. In addition, the invention is by no means restricted to operation with an electromagnetic relay. Switches 15a, 15b and may, for example, be entirely mechanically operated through a mechanical timer device and the like. The techniques underlying the present invention, accordingly, are completely adaptable for use with purely mechanical vibrating systems, as well as the electrical systems before referred to. Similarly, other types of electro-mechanical switching mechanisms may be employed to attain the fail-sale results achieved in accordance with the present invention.

Further modifications will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. In an electrical system in which a principal signal modified by a repetitive checking signal having a predetermined rate or rates of repetition is transmitted from the input to the output of the system, an output circuit comprising switching means respnsive to the recovery of the checking signal from the principal signal in the output and adapted repetitively to occupy alternate positions at the said repetition rate or rates, means for supplying direct-current potential of a predetermined magnitude, potential-storing means, an electric circuit operative when the switching means occupies one of its positions to connect together the supplying means and the storing means to store direct-current potential in the storing means, a slow-response direct-current-operated load adapted to respond after the elapse of a period greater than the period or periods of the said repetition rate or rates and operable only in response to direct-current potential of magnitude greater than the said predetermined magnitude, and a further electric circuit operative when the switching means occupies the alternate position to connect the potentialstoring means and the supplying means in series and to the load.

2. In an electrical system in which a principal signal modified by a repetitive checking signal having a predetermined rate or rates of repetition is transmitted from the input to the output of the system, an output circuit comprising switching means responsive to the recovery of the checking signal from the principal signal in the output and adapted repetitively to occupy alternate positions at the said repetition rate or rates, means for supplying alternating-eurrent potential, means for converting alternating-current potential into direct-current potential, first and second potential-storing means, an electric circuit operative when the switching means occupies one of its positions to connect together the alternatingcurrent supplying means, the converting means and the first and second potential-storing means to store directcurrent potential of a predetermined magnitude in each of the first and second potential-storing means, a slowresponse direct-current operated-load adapted to respond after the elapse of a period greater than the period or periods of the said repetition rate or rates and operable only in response to direct-current potential of magnitude greater than the said predetermined magnitude, and a further electric circuit operative when the switching means occupies the alternate position to connect the first and second potential-storing means in series and to the load.

3. In an electrical system in which a principal signal is transmitted from the input to the output of the system, an output circuit comprising switching means responsive to the reception of the principal signal in the output for alternately first reacting upon the principal signal in the input to modify the same and then responding to the resulting modification in the output to restore the principal signal at the input, thereby to produce a repetitive checking-signal modification of the principal signal at a predetermined rate or rates of repetition, the switching means occupying alternate positions during such operation,

7 means for supplying direct-current potential of a predetermined magnitude, potential-storing means, an electric circuit operative when the switching means occupies one of its positions to connect together the supplying means and the storing means to store direct-current potential in the storing means, a slow-response direct-current-operated load adapted to respond after the elapse of a period greater than the period or periods of the said repetition rate or rates and operable only in response to direct-current potential of magnitude greater than the said prede termined magnitude, and a further electric circuit operative when the switching means occupies the alternate position to connect the potential-storing means and the supplying means in series and to the load.

4. In an electrical system in which a principal signal is transmitted from the input to the output of the system, an output circuit comprising switching means responsive to the reception of the principal signal in the output for alternately first reacting upon the principal signal in the input to modify the same and then responding to the resulting modification in the output to restore the principal signal at the input, thereby to produce a repetitive checking-signal modification of the principal signal at a predetermined rate or rates of repetition, the switching means occupying alternate positions during such operation, means for supplying alternating-current potential, means for converting alternating-current potential into direst-current potential, first and second potential storing means, an electric circuit operative when the switching means occupies one of its positions to connect together the alternating-current supplying means, the converting means and the first and second potential-storing means to store direct-current potential of a predetermined magnitude in each of the first and second potential-storing means, a slow-response direct-current operated load adapted to respond after the elapse of a period greater than the period or periods of the said repetition rate or rates and operable only in response to direct-current potential of magnitude greater than the said predetermined magnitude, and a further electric circuit operative when the switching means occupies the alternate position to connect the first and second potential-storing means in series and to the load.

5. In an electrical system in which a principal signal modified by a repetitive checking signal having a predetermined rate or rates of repetition is transmitted from the input to the output of the system, an output circuit comprising relay-controlled switching means responsive to the recovery of the checking signal from the principal signal in the output and adapted repetitively to occupy alternate positions at the said repetition rate or rates, means for supplying direct-current potential of a predetermined magnitude, capacitor means, an electric circuit operative when the switching means occupies one of its positions to connect together the supplying means and the capacitor means to store direct-current potential in the capacitor means, a direct-current-operated capacitorshunted load relay adapted to respond after the elapse of a period greater than the period or periods of the said repetition rate or rates and operable only in response to direct-current potential of magnitude greater than the said predetermined magnitude, and a further electric circuit operative when the switching means occupies the alternate position to connect the capacitor means and the supplying means in series and to the capacitor-shunted load relay.

6. In an electrical system in which a principal signal is transmitted from the input to the output or" the system, an output circuit comprising relay-controlled switching means responsive to the reception of the principal signal in the outputfor alternately first reacting upon the principal signal in the input to modify the same and then responding to the resulting modification in the output to restore the principal signal at the input, thereby to produce a repetitive checking-signal modification of the principal signal at a predetermined rate or rates of repetition, the switching means occupying alternate positions during such operation, means for supplying direct-current potential of a predetermined magnitude, capacitor means, an electric circuit operative when the switching means occupies one of its positions to connect together the supplying means and the capacitor means to store directcurrent potential in the capacitor means, a direct-currentoperated capacitor-shunted load relay adapted to respond after the lapse of a period greater than the period or periods of the said repetition rate or rates and operable only in response to direct-current potential of magnitude greater than the said predetermined magnitude, and a further electric circuit operative when the switching means occupies the alternate position to connect the capacitor means and the supplying means in series and to the capacitor-shunted load relay.

7. In an electrical system in which a principal signal modified by a repetitive checking signal having a predetermined rate or rates of repetition is transmitted from the input to the output of the system, an output circuit comprising switching means responsive to the recovery of the checking signal from the principal signal in the output and adapted repetitively to occupy alternate positions at the said repetition rate or rates, means for supplying alternating-current potential, means for converting alternatingcurrent potential into direct-current potential, first and second potential-storing means, an electric circuit operative when the switching means occupies one of its positions to connect together the alternatin -current supplying means, the converting means and the first and second potential-storing means with the storing means connected in parallel with each other to store direct-current potential of a predetermined magnitude in each of them, a slowresponse direct-current operated load adapted to respond after the elapse of a period greater than the period or periods of the said repetition rate or rates and operable only in response to direct-current potential of magnitude greater than the said predetermined magnitude, and a further electric circuit operative when the switching means occupies the alternate position to connect the first and second potential-storing means in series and to the load and substantially simultaneously to disconnect the storing means from the alternating-current supplying means.

8. An electric system having, in combination, switching means adapted to occupy alternate positions, means for repetitively operating the switching means between its positions at a predetermined repetition rate or rates, means for supplying direct-current potential of a predetermined magnitude, potential-storing means, an electric circuit operative when the switching means occupies one of its positions to connect together the supplying means and the storing means to store direct-current potential in the storing means, a slow-response direct-current-operated load adapted to respond after the elapse of a period greater than the period or periods of the said repetition rate or rates and operable only in response to direct-current potential of magnitude greater than the said predetermined magnitude, and a further electric circuit operative when the switching means occupies the alternate position to connect the potential-storing means and the supplying means in series and to the load.

9. An electrical system having, in combination, switching means adapted repetitively to occupy alternate positions at a predetermined repetition rate or rates, means for supplying direct-current potential of a predetermined magnitude, potential-storing means, an electric circuit operative when the switching means occupies one of its positions to connect together the supplying means and the storing means to store direct-current potential in the storing means, a slow-response direct-current-operated load adapted to respond after the elapse of a period greater than the period or periods of the said repetition rate or rates and operableonly in response to directcurrent potential of magnitude greater than the said predetermined magnitude, and a further electric circuit operative when the switching means occupies the alternate position to connect the potential-storing means and the supplying means in series and to the load.

10. An electric system having, in combination, switch ing means adapted to occupy alternate positions, means for repetitively operating the switching means between its positions at a predetermined repetition rate or rates, means for supplying alternating-current potential, means for converting alternating-current potential into direct-current potential, first and second potential-storing means, an electric circuit operative when the switching means occupies one of its positions to connect together the alternating-current supplying means, the converting means and the first and second potential-storing means to store direct-current potential of a predetermined magnitude in each of the first and second potential-storing means, a slow-response directcurrent-operated load adapted to respond after the elapse of a period greater than the period or periods of the said repetition rate or rates and operable only in response to direct-current potential of magnitude greater than the said predetermined magnitude, and a further electric circuit operative when the switching means occupies the alternate position to connect the first'and second potential-storing means in series and to the load.

11. An electric system having, in combination, switching means adapted to occupy alternate positions, means for repetitively operating the switching means between its positions at a predetermined repetition rate or rates, means for supplying alternating-current potential. means for converting alternating-current potential into direct-current potential, first and second potential-storing means, an electric circuit operative when the switching means occupies one of its positions to connect together the alternatingcurrent supplying means, the converting means and the first and second potential-storing means with the storing means connected in parallel with each other to store directcurrent potential of a predetermined magnitude in each of them, a slow-response direct-current operated load adapted to respond after the elapse of a period greater than the period or periods of the said repetition rate or rates and operable only in response to direct-current potential of magnitude greater than the said predetermined magnitude, and a further electric circuit operative when the switching means occupies the alternate position to connect the first and second potential-storing means in series and to the load and substantially simultaneously to disconnect the storing means from the alternating-current supplying means.

12. An electrical system having, in combination, switching means adapted repetitively to occupy alternate positions at a predetermined repetition rate or rates, means for supplying alternating-current potential, means for converting alternating-current potential into direct-current potential, first and second potential-storing means, an electric circuit operative when the switching means occupies one of its positions to connect together the alternating-current supplying means, the converting means and the first and second potential storing means to store direct-current potential of a predetermined magnitude in each of the first and second potential-storing means, a slow-response directcurrent operated load adapted to respond after the elapse of a period greater than the period or periods of the said repetition rate or rates and operable only in response to direct-current potential of magnitude greater than the said predetermined magnitude, and a further electric c-ircuit operative when the switching means occupies the alternate position to connect the first and second potential-storing means in series and to the load.

13. An electrical system as claimed in claim 9 and in which the switching means is relay-controlled, the potential-storing means comprises a capacitor and the load comprises a slow-response relay.

14. An electrical system as claimed in claim 12 and in which the switching means is relay-controlled, the first and second potential-storing means comprise capacitors and the load comprises a slow-response relay.

15. A fail-safe system for preventing false effective energization of an electrical load through integrity failure of any component of the system having, in combination, voltage-supply terminals adapted to be energized with direct-current potential of predetermined magnitude, switching means having multiple positions, a capacitor arranged to be energized from the said potential when the switching means is disposed in a predetermined position, a slowly de-energizable load adapted to be efiectively energized only when connected to direct-current potential of magnitude in excess of the said predetermined magnitude, the said switching means being arranged to connect the said load, the said capacitor and the said voltagesupply terminals all in series connection when in another predetermined position, thereby to effectively energize the load, and means for recurrently operating the said switching means between the said predetermined positions at a predetermined frequency or frequencies to maintain efiective energization of the load.

16. A fail-safe system for preventing false effective energization of an electrical load through integrity failure of any component of the system having, in combination, voltage-supply terminals adapted to be energized with alternating-current potential, switching means having multiple positions, first and second capacitors, rectifier means, an electric circuit adapted to connect together the voltage-supply terminals, the rectifier and the first and second capacitors to energize each of the capacitors with direct-current potential of a predetermined magnitude when the switching means is in a first predetermined position, a slowly de-energizable load adapted to be efiectively energized only by a direct-current voltage potential of magnitude in excess of the said predetermined magnitude, the said switching means being arranged to connect the first capacitor, the second capacitor and the load all in series connection when in another predetermined position, thereby effectively to energize the load, and means for recurrently operating the said switching means between its said positions at a predetermined frequency or frequencies in order to suppress de-energization of the oad.

References Cited in the file of this patent UNITED STATES PATENTS 

